Push-pull amplifying system for ultra-high frequencies



Nov. 30, 1943. A. VAN DER'ZIEL ET AL 2,335,820

PUSH-PULL AMPLIFYING SYSTEMS FOR ULTRA HIGH FREQUENCIES I Filed May 9, 1941 T A. T 5& RH Y O S E TR N N R E J o VN N mm A m WM Mm M M M Patented Nov. 30,. 1943 PUSH-PULL AIVIPLIFYING SYSTEM FOR ULTRA-HIGH FREQUENCIE S.

Aldert van der Ziel and Maximiliaan Julius Otto Strutt, Eindhoven, Netherlands; vested in the Alien Property Custodian Application May 9, 1941, Serial No. 392,634 In the Netherlands June 27, 1940 7 Claims. '(Cl. 179-1'l1) The invention relates to an amplifying system for ultra-high frequencies which. comprises two amplifying systems in push-pull connection which are each constituted at least by a cathode, a control grid, a screen grid and an anode and it has for its purpose to provide means for neutralizing in such a system the retroaction of the anode voltageon the control-grid circuit.

With the usual broadcast frequencies this retroaction is almost exclusively caused by the anode-control grid capacity Cag. With higher frequencies the self-inductions and the mutual inductions of the supply conductors leading to the tube electrodes also 'play a part with the resultthat the phenomena become much more complicated. However, for these higher frequencies, the retroaction may still be imagined to be brought about by a capacity Gag which is operative between the anode and the control grid and for which we may write frequencies above the" critical frequency there occurs between the anode and the control grid 1 an effective negative capacity the value of which may be considerably larger than that of the static anode-control grid capacity Cag- With screen-grid tubes the static anode-control grid capacity is as arule very slight (for example about 0.003 ##F) so that at frequencies below the criticalfrequency there is in general no need for neutralization of the retroaction. At frequencies exceeding the critical frequency, how-.

ever, the retroaction may be very appreciable also with screen-grid tubes. Fundamentally it would in this case be possible to neutralize the retroaction by artificially increasing the anodecontrol grid capacity, that is to say by arrang ng a regulable condenser between the anode and the. .control gridand by adjusting this condenser in such manner that. the resulting capacity between the anode-and the control grid is equal to 02A. In practice, however this method cannot be carried into effect since an adjustable condenser of the very low capacity required therefor is impracticable.

It is known to neutralize the retraction with a push-pull amplifying system by connecting the anode of each of the amplifying tubes via an adjustable condenser to the control grid of the other tube. This known method only permits, however, to neutralize a positive anode-control grid capacity so that with frequencies exceeding the critical frequency it is impracticable.

The invention has. for its object to provide means for neutralizing the retroaction in a pushpull amplifying system, with which the frequency of the oscillations to be amplified exceeds the critical frequency.

According to the invention, the retroaction exerted by the anode voltage on the control-grid circuit is neutralized in each of the amplifying systems by connecting a point of the circuit of one of the other electrodes of the amplifying system in question through an adjustable condenser to the anode of the other amplifying system, a selfinduction being present in the high-frequency connection between the said point and the midpoint of the input circuit.

Preferably, a point of the screen-grid circuit of each of the amplifying systems is connected through an adjustable condenser to the anode of the other amplifying system.

The invention will be explained more fully with reference to the accompanying drawing in which Figs. 1 and 2 represent, by way of example, two embodiments thereof, and Fig. 3 is a diagram by the aid of which the operation of the circuits ,of Figs. 1 and 2 will be explained, the diagram showing the principal impedances of said circuits which play a part in the production and neutralization of retroaction.

Fig. 1 represents a push-pull amplifying tube l compris ng two'amplifying systems the upper one of which consists of a cathode 2, a control grid 3, a screen grid 4, a suppressor grid 5 connected to the cathode and an anode 6 whilst the other amplifying system consists of similar electrodes denotedby primed reference numerals. In order to reduce to a minimum the self-induction of that part of the cathode lead which carries alternating current, which self-induction gives rise to damping of the input circuit, the two cathodes 2 and 2' are preferably formed as one unit or connected to one another by means of a conductor which is as short as possible.

An input oscillatory circuit 1 is connected in push-pull connection to the control grids 3 and 3'.

The oscillations to be amplified are su lied to terminals 8 and 9 and inductively transmitted to the circuit I. Between the anodes 6 and 6 is provided an output oscillatory circuit Ill the ends of which are connected, through the intermediary of separating condensers, to output terminals H and E2. The mid-point of the circuit l and the mid-point of the circuit l!) are con nected to the cathodes 2 and 2 through condensers which form a short-circuit for the frequency of the oscillations to be amplified.

. The anode t of the upper amplifying system is connected through an adjustable neutralizing condenser 83 to the screen grid 4 of the lower amplifying system whilst the anode 6' of the lower amplifying system is connected through a similar condenser [4 to the screen grid 4 of the upper amplifying system. Between the screen grids 4 and 4' and the cathode are provided selfinductances l and it.

If the frequency of the oscillations to be amplified exceeds the critical frequency of the amplifying systems the exact adjustment the condensers i3 and i4 permits to neutralize the retroaction completely.

The function of the inductances l5 and It may also be performed by the natural self-inductions of the supply conductors of the screen grids 4 and 4 as is shown in Fig. 2. To that end the two screen grids must each be provided with two separated supply conductors, each screen grid being connected through one of these supply conductors to the cathode of the amplifying system in question and through the other supply conductor via a neutralizing condenser to the anode of the other amplifying system.

The operation of the systems according to Figs. 1 and 2 will be explained more'fully with reference to Fig. 3 which represents a simplified substitution diagram which only exhibits the principal impedances which play a part in the production and in the neutralization of the retroaction. This substitution diagram applies to one of the amplifying systems; for the other amplifying system may naturally be plotted a similar substitution diagram.

In Fig. 3 the anode-control grid capacity is denoted by Cag, the anode-screen grid capacity by Gas and the screen grid-control grid capacity by Csg. That portion ofthe input impedance which is located between the control grid 3 and the cathode is denoted by Z1 whilst Cn denotes the capacity 'of the neutralizing condenser [4.

For. the system according to Fig. 1, L1 is the natural self-induction of the supply conductor of the screen grid 4 whilst L2 denotes the selfinduction of the coil IS. with the system according to Fig. 2, L1 may be considered equal to zero (since it is in series with condenser l4 solely and hence merely affects the apparent capacity of l4 slightly), whilst L2 represents the natural self-induction of the supply conductor of the screen grid 4, which conductor is connected to the cathode. The anode alternating voltage of the upper amplifying system is denoted by Ea, that of the lower amplifying system by --Ea. The arrows indicate the direction of the currents flowing in the various impedances.

The substitution diagram according to Fig. 3 is in so far incomplete that in practice also the self-inductions of the supply conductors leading to the suppressor grid and the cathode and the capacities of these electrodes with respect to the anode, the screen grid and the control grid as well as the mutual inductions of the supply conductors may play a part. These impedances, however, are in practice of less importance.

The impedances of the capacities Gas, Gas, Csg and Cu are in practice high with respect to Zi and to the impedances of the self-inductances L1 and L2. With complete neutralization the voltages Ea and Ea cause no voltage on the control grid so that the control grid alternating voltage may be assumed to be equal to zero.

Roughly stated the following values of currents I and volt'ages may be assumed.

A current jwCag-Ea fiows through the capacity Cag to the control grid. 4

Through the capacity Cas flows to the screen grid a current jwCasJila which sets up across the self-inductances L1 and L2 a voltage iilhis voltage furnishes a current flowing to the control grid through the capacity 'Csg,

which current is equal to 7'w CasC'sg(L1-l-Lz).Ea v

jw CnCs LzEa and is consequently in phase with the current flowing ,through the capacity Gag to the control grid.

If CnLZ is made equal to Cas(Ll+L2) the retroaction exerted through the screen grid is completely suppressed so that there only remains the retroaction exerted via the small capacity Cag. If it is desired to neutralize also the latter retroaction, On has to be taken slightly smaller. For

the case of Fig. 2 it is consequently necessary to take for neutralization On equal to or slightly smaller than 0:15.

In the circuit arrangements according to Figs. 1 and 2 a point of the screen-grid circuit of each of the amplifying systems is connected through a neutralizing condenser to the anode. Fundamentally it is also possible to choose therefor a point of the circuit of another electrode, for example of the cathode. In practice, however, the latter method is not recommended since in connection with the input damping brought about by the self-induction of the'cathode lead this self-induction is preferably kept as small as possible.

What we claim is:

1. An amplifying system for ultra-high frequencies comprising two amplifying systems connected in push-pull and each constituted at least by a cathode, a control grid, a screen grid and an anode, characterized in that the retroactionj exerted by the anode voltage on the control-grid ing systems is provided with two separated supply conductors and the screen grid is connected through the intermediary of one of these supply conductors to the cathode of the amplifying system in question and through the intermediary of the other supply conductor via the said adjustable condenser to the anode of the other amplifying system.

3. An amplifying system as claimed in claim 1, wherein the screen grid of each of the amplifying systems is provided with two separated supply conductors and the screen grid is connected through the intermediary of one of these supply conductors to the cathode of the amplifying system in question and through the intermediary of the other supply conductor via the said adjustable condenser to the anode of the other amplifying system, and wherein the impedance present between the said point and the mid-point of the input circuit is formed by the natural self-induc- .tion of the supply conductor through the intermediary of which the screen grid is connected to the cathode.

4. A circuit for the amplification of ultra-short waves comprising a push-pull tube provided with two electrode systems each of which has at least a cathode, a signa1 control grid, a screen grid and an anode, the cathodes being connected together by a lead as short as possible, an input circuit connected to the signal control grids, an output circuit connected between the anodes, a high frequency circuit connected'between each screen grid and the common cathode lead, and means for neutralizing the feed-back between the anode and signal grid of each electrode system, said means comprising a pair of condensers which are cross-connected between a point in the screen grid circuit of one electrode system and the anode of the other.

5. A circuit for the amplification of ultra-short waves comprising a push-pull tube provided with two electrodesystems each of which has at least a cathode, a signal control grid, a screen grid and an anode,'the cathodes being connected together by a lead as short as possible, an input circuit connected to the signal control grids, an outputcircuit connected between the anodes, and means for neutralizing the effective feed-back from the output to input circuits due to interelectrode capacities, said means comprising a pair of condensers which are cross-connected directly between the screen grid of one electrode system and the anode of the other.

6. A circuit for the amplification of ultra-short waves comprising a push-pull tube provided with two electrode systems each of which has at least a cathode, a signalcontrol grid, a screen grid and an anode, the cathodes being connected together by a lead as short as possible, an input circuit connected to the signal control grids, an output circuit connected between the anodes, an inductance connected between each screen grid and the common cathode lead, and a pair of neutralizing condensers which are cross-connected between the anode of one electrode system and the screen grid end of the inductance connected to the screen grid of the other electrode system.

7. A circuit for the amplification of ultra-short waves comprising apush-pull tube provided with two electrode systems each of which has at least a cathode, a signal control grid, a screen grid and an anode, the cathodes being connected together by a lead as short as possible, an input circuit connected between the signal control grids, an output circuit connected between the anodes, a pair of supply leads connected to each screen grid, a connection from one of the supply leads of each screen grid to the common cathode lead, and a. pair of neutralizing condensers crossconnected between the other supply lead of a screen grid of one electrode system and the anode of the other electrode system.

ALDERT VAN nan ZIEL. v MAXIMILIAAN JULIUS OTTO STRUT'I. 

